1. Field of the Invention
The present invention relates to a two cycle engine, and more particularly to a two cycle engine which is suitable for a handheld tools powered by an engine such as a bush cutter, a chain saw, a blower and the like, and a two-cycle-engine-powered tool provided with the two cycle engine.
2. Description of the Related Art
Conventionally, there have been proposed a lot of inventions which relate to a two cycle engine reducing an outflow (blow-by) of an air-fuel mixture within a cylinder into an exhaust port, in a scavenging process.
For example, in Patent Literature 1, there is proposed a stratified scavenging two cycle engine which scavenges a combustion gas by making a lead air having flown into an internal space of piston through a lead air port and a lead air flow path in a suction stroke, and making the lead air flowing into the internal space of piston flow into a cylinder through a scavenge air communication port, a scavenge air inflow port, a scavenge air passage and a scavenge air port in a scavenging stroke.
Patent Literature 1: Unexamined Japanese Patent Application KOKAI Publication No. 2008-14209
As mentioned above, since the stratified scavenging two cycle engine proposed in the Patent Literature 1 scavenges the combustion gas by the lead air, it is possible to reduce the blow-by of the air-fuel mixture in the scavenging stroke. However, since a pressure within a cylinder (a combustion chamber) is higher than a pressure within the internal space of piston in an early stage of the scavenging stroke, there is a problem that the combustion gas within the cylinder flows backward to the internal space of piston through the scavenge air port, the scavenge air passage, the scavenge air inflow port, and the scavenge air communication port, and further intrudes into a crank chamber. Moreover, if the combustion gas makes an intrusion into the crank chamber, a concentration of the air-fuel mixture within the crank chamber is lowered. As a result, there is a problem that an output of an engine is lowered, and an amount of total hydrocarbons (THC) included in a discharge gas is increased.